Nitrogen rejection method and apparatus

Abstract

Nitrogen is rejected from a feed gas stream comprising methane and nitrogen so as to form a methane product. The feed gas stream is cooled in a main heat exchanger and rectified in a double rectification column comprising a higher pressure column, a lower pressure column and a condenser-reboiler placing the higher and lower pressure rectification columns in heat exchange relationship with each other. At least part of the feed gas stream is expanded through a valve into the higher pressure rectification column and the feed gas stream is partially liquefied upstream of the double rectification column. Over a period of time, the mole fraction of nitrogen in the feed gas stream may increase. The operating pressure of the lower pressure rectification column is periodically increased in response to increases in the mole fraction of nitrogen. A back pressure valve may be periodically adjusted to effect the increase in pressure.

What is claimed is:  
     
       1. A method of rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, the mole fraction of nitrogen in the feed gas increasing over a period of time, the method comprising cooling the feed gas stream in a main heat exchanger, rectifying the cooled feed gas stream in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, expanding at least part of the feed gas stream into the higher pressure rectification column, partially liquefying the feed gas stream upstream of the double rectification column, and periodically increasing the operating pressure of the lower pressure rectification column in response to increases in the mole fraction of nitrogen in the feed gas stream. 
     
     
       2. The method according to  claim 1 , wherein a first flow of gas is passed as a first recycle gas flow from the lower pressure rectification column to the higher pressure rectification column. 
     
     
       3. The method according to  claim 2 , wherein the first recycle gas flow is compressed, is cooled in the main heat exchanger, is liquefied in the condenser-reboiler, and is introduced into the higher pressure rectification column. 
     
     
       4. The method according to  claim 2 , wherein a product methane stream is withdrawn in liquid state from the lower pressure rectification column, is raised in pressure, and is vaporised at least in part in the main heat exchanger. 
     
     
       5. The method according to  claim 2 , wherein a second recycle gas flow is taken from the lower pressure rectification column, is compressed, is cooled in the main heat exchanger, and is introduced at least partly in liquid state into the top of the higher pressure rectification column. 
     
     
       6. The method according to  claim 2 , wherein a second recycle gas flow is taken from the lower pressure rectification column, is compressed, is cooled in the main heat exchanger, and is introduced at least partly in liquid state into the top of the higher pressure rectification column and the second recycle gas flow is compressed to a higher pressure than the first recycle gas flow. 
     
     
       7. The method according to  claim 2 , wherein a second recycle gas flow is taken from the lower pressure rectification column, is compressed, is cooled in the main heat exchanger, and is introduced at least partly in liquid state into the top of the higher pressure rectification column, and the first and second recycle gas flows are compressed in a plural stage compressor, the second recycle flow being taken from a stage of the compressor downstream of a stage from which the first recycle flow is taken. 
     
     
       8. The method according to  claim 2 , wherein the first recycle gas flow is compressed before being passed to the higher pressure rectification column, and a vent stream is taken from the first recycle gas flow upstream of its compression and is vented. 
     
     
       9. The method according to  claim 1 , wherein the pressure in the lower pressure rectification column is adjusted so as to maintain a constant percentage recovery of methane in the product gas. 
     
     
       10. The method according to  claim 1 , wherein all the bottom fraction obtained in the higher pressure rectification column is withdrawn therefrom and is sent to the low pressure rectification column. 
     
     
       11. Apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, the apparatus comprising a main heat exchanger for cooling the feed gas stream, a double rectification column for rectifying the feed gas stream comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, and an expansion device downstream of the main heat exchanger communicating with the higher pressure rectification column, the expansion device being arranged for introducing a part of the feed gas stream into the higher pressure rectification column in liquid state, wherein there is a back pressure regulating valve associated with the lower pressure rectification column which is operable to increase the pressure in the lower pressure rectification column. 
     
     
       12. The apparatus according to  claim 11 , wherein the back pressure regulating valve is able to be controlled so as to maintain a constant percentage recovery of methane in the product gas.